The present invention generally relates to a wafer positioning apparatus, and more particularly to a wafer positioning apparatus for positioning a wafer which employs photosensors and a transportation robot.
A wafer positioning apparatus is employed in equipment which handles a wafer such as a semiconductor wafer. For example, when a wafer is accommodated into a carrier station, it is necessary to position the wafer so as to prevent the wafer from coming into collision with the carrier station and being damaged. After the wafer is positioned, it is transported to the carrier station, and is then accommodated therein. A wafer positioning apparatus is an apparatus for enabling the above positioning operation.
A conventional wafer positioning apparatus includes two opposed wafer holding members, which are located so that a wafer is interposed therebetween. At the time of positioning the wafer, the holding members are driven so as to make pressure contact with and hold opposed circumferential portions of the wafer. However, the above wafer positioning apparatus may damage circumferential portions of the wafer and may deform the wafer.
In order to overcome the above problems, another wafer positioning apparatus has been proposed in the Japanese Laid-Open Patent Application No.60-85536. The disclosed wafer positioning apparatus includes a suction holder and at least three optical position detectors. Each of the optical position detectors consists of a light-emitting diode and a photodetector, which are arranged so as to be opposite to each other. When the wafer becomes positioned under the light-emitting diode, the photodetector is prevented from receiving the light emitted from the light-emitting diode.
Two of the three optical position detectors are arranged along an orientation flat portion formed in a wafer which is a straight peripheral portion of the wafer. The two optical position detectors are spaced at a distance shorter than the length of the orientation flat portion. When the two optical position detectors detect the orientation flat portion, and simultaneously the third position detector detects the circumferential portion of the wafer, the wafer positioning operation is completed. In order to obtain the above state, the wafer, held by the suction holder, is moved in X and Y directions and is rotated in accordance with signals supplied from the optical position detectors.
However, the proposed wafer positioning apparatus has disadvantages described below. First, it is difficult to fabricate a compact wafer positioning apparatus, because a mechanism for rotating the wafer around a rotation axis by at least one rotation is necessary to position the orientation flat portion under the two optical position detectors for detecting the orientation flat portion. Secondly, the proposed wafer positioning apparatus cannot position a wafer which does not have an orientation flat portion.